go with the 18i20.. yan ang gamit ko ngayon dito sa studio ko... i have the second generation.. still usb 2.0 tho.. so far i havent experienced anything bad sa 18i20.. ginagamit ko sya with cubase 5 pa.. no conflicts so far kahit obsolete na yung daw ko.. i would suggest buying a snake cable or a simple xlr patchbay since the 18i20 is designed to be placed in a rack and the rest of the i/o's are at the back.. you would find a hard time plugging and unplugging instruments or mics when its already set up in your rack..

but if you really want low latency mas maganda mag focusrite clarett ka na lang.. thunderbolt yun.. halos walang latency.. you could even run realtime na effects.. most of the pc's today already have thunderbolt natively.. sabi sa website ng focusrite they would be releasing drivers for pc na rin for the clarett any time now..

I use Saffire Pro 40 without problems and with good results, but I recommend Scarlett 18i20. Firewire is obsolete tech, so it's really hard to find a firewire card or even firewire cables. USB is a more persistent tech.

Focusrite has developed zero-latency monitoring by bypassing your PC (your monitor goes from the interface direct to your speaker or headphones, instead of going to your PC, back to the interface, and into your speakers or headphones). Both models has this feature. Latency only appears when you monitor along with your DAW's effects. E.g., you can't hear your voice with reverb as you are recording, but reverb can be applied after you record, in post.